Effect of PIP₂ on Bilayer Structure and Phase Behavior of DOPC: An X‐ray Scattering Study

Abstract

Phosphatidylinositol 4,5-bis-phosphate (PIP(2)) is an important lipid in regulation of several cellular processes, particularly membrane fusion. We use X-ray diffraction from solid-supported multilamellar 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/PIP(2) samples to study changes in bilayer structure and the lyotropic phase behavior induced by physiologically relevant concentrations of PIP(2) . Electron-density profiles reconstructed from X-ray reflectivity measurements indicate that PIP(2) strongly affects structural parameters such as lipid head-group width, bilayer thickness, and lamellar repeat spacing of DOPC bilayer stacks. In addition, at lower degrees of hydration, a few molar per cent of PIP(2) facilitates stalk-phase formation and also leads to formation of a hexagonal phase, which is not observed in pure DOPC. These results indicate that the role of PIP(2) in membrane fusion could be, in part, due to its effect on the properties of the lipid bilayer matrix. Furthermore, coexistence of two lamellar phases with different lattice constants is observed in single-component PIP(2) samples.

Energetics of stalk intermediates in membrane fusion are controlled by lipid composition

“…With additional control parameters (i.e., osmotic pressure, temperatature, composition) a transition to a nonlamellar states is then easily induced. In this context, we also found that the lipid phosphatidylinositol-4,5-bisphosphate (PIP 2 ), which is enriched at the sites of vesicle fusion of synaptic membranes (84,85), has a very pronounced effect on the phase behavior of DOPC and promotes stalk and inverted hexagonal phase formation already at concentrations of few mol% (43). As a strongly anionic lipid with three net negative charges, PIP 2 is likely to interact with Ca 2þ , possibly again facilitating localized dehydration and hydrophobic defect formation.…”

“…2B) were then reconstructed by aid of the swelling method for lamellar phases described e.g., in ref. 43 and references therein. Following the well established osmotic stress method (31,33,42), it is assumed that the total attractive pressure can be approximated by the osmotic pressure P ¼ −ðk B T ∕v w Þ lnðRH∕100%Þ which dominates the van der Waals attraction in the used RH range (33,88).…”

Interbilayer repulsion forces between tension-free lipid bilayers from simulation

“…1,35,37 The applied protocols are described elsewhere in detail. 21,38,39 In brief, stacks of about 1500 aligned lipid bilayers were prepared on silicon substrates and placed in an environmental chamber with precise RH control. 39 The latter allowed us to tune the osmotic pressure .…”

“…ref. 38 and 41 and references therein) as follows: rstly, due to the nonmonotonous curve d(RH), only data for RH > 85% in the regime of "normal" swelling were used (Fig. 1b).…”

Stalk formation as a function of lipid composition studied by X-ray reflectivity

“…According to molecular dynamics (MD) simulations, PIP 2 changes the structural properties of the host lipid bilayer by rearranging and reorganizing its surrounding lipids to form stable microdomains [31]. The phase diagram of PC/PIP 2 shows that it can promote non-lamellar phases [14]. At low concentration, PIP 2 promotes the stalk-phase, and at higher concentration the formation of the hexagonal phase, which is usually not observed for pure DOPC.…”